Solar cells are desired to have electrodes with a high aspect ratio. The desired electrode may be formed by screen printing an electrode paste to form a first electrode layer, and repeating screen printing to lay plural electrode layers thereon. This method is known as multilayer electrode printing. To carry out accurate multilayer electrode printing, alignment marks are utilized as shown in FIGS. 1A-1C Alignment marks are formed on a wafer or workpiece subject to printing as shown in FIG. 2. Specifically, FIG. 2 illustrates an exemplary solar cell having two alignment marks. Disposed on a semiconductor substrate 11 are finger electrodes 12, bus bar electrodes 13, and alignment marks 14. First, the shape and location of initial alignment marks (A0, B0 in FIG. 1A) are previously stored in an image processing device. Secondly, alignment marks (A1, B1 in FIG. 1B) of the same shape as the initial alignment marks are printed on the substrate at the same time as is a first layer of electrode. Thirdly, the workpiece having marks A1, B1 printed is placed on a stage for printing second and subsequent layers of electrode. Fourthly, marks A1, B1 are recognized by cameras, and the stage position is finely adjusted so that marks A1, B1 may overlay the positional data of marks A0, B0 of the same shape. Thereafter, electrode paste is printed. In this way, a multilayer electrode can be formed.
While printing is repeated, there can occur changes in viscosity of the electrode paste or fissure of the workpiece, whereupon accurate alignment marks are no longer printed. There result alignment marks A2 and B2 in FIG. 1C. Now that the difference in shape between the mark actually printed and the mark previously stored in the image processing device is of significance, accurate image processing becomes impossible. As a result, the lower layer of electrode and the overlying layer of electrode printed thereon are shifted to increase a shadow loss, detracting from the conversion of the solar cell.
JP-A 2006-080415, for example, addresses the problem in that alignment marks formed by screen printing are image processed for positional correction and observed for any anomaly at the same time. When an anomaly is detected, the process is interrupted. The method of JP-A 2006-080415, however, does not address changes of alignment marks with time (e.g., oozing due to lowering of paste viscosity, incidental flaws). Since the process is interrupted whenever an anomaly is detected in the mark, the down time is a problem.